R - Produce a string that summarizes a vector of integers by replacing sequential values with the start and end value of the sequence

Question

I am looking for an efficient way to simplify a vector of integers as a summary string, in order to format it to fit in a table cell.

For example:

c(1, 2, 3, 4, 6, 8, 9, 10)

should produce

"1-4, 6, 8-10"

This becomes especially useful in cases where printing all elements in the vector would quickly make the table unreadable.

e.g.

c(1:50, 53, 89:120)

should produce

"1-50, 53, 89-120"

Answer 1

You want to group the elements into blocks of consecutive integers. diff can tell you if two consecutive elements are in the same block, cumsum can number the blocks and tapply can extract the first and last element of each block.

x <- c(1:50, 53, 89:120)
y <- tapply( x, c(0,cumsum(diff(x) != 1)), range )
# Format the result
y <- sapply(y, function(u) 
  if(u[1]==u[2]) u[1] 
  else paste(u,collapse=":")
)
paste(y, collapse=", ")